The oxygen sensor fumarate nitrate reductase regulator (FNR) of Escherichia coli contains in the active (anaerobic) state a [4Fe-4S](2+) cluster which is lost after exposure to O(2). In aerobically prepared apoFNR, or in FNR obtained by treatment of [4Fe-4S] . FNR with O(2) in vitro, intramolecular cysteine disulfides are found, including the cysteine residues which serve as ligands for the Fe-S cluster. It is shown here that the reconstitution of [4Fe-4S] . FNR from this form of aerobic apoFNR was preceded by a long lag phase when glutathione was used as the reducing agent. Addition of E. coli glutaredoxins (Grx) 1, 2 or 3 decreased the lag phase greatly and stimulated the reconstitution rate slightly (about twofold). Reconstitution of anaerobically prepared apoFNR, which has a lower cysteine disulfide content, showed only a short lag phase, which further decreased in the presence of Grx. It is concluded that in the lag phase the cysteine disulfides of apoFNR become reduced for the incorporation of the [4Fe-4S] cluster and that this reaction is stimulated by Grx. Thioredoxin (Trx) 1 showed no stimulation of FNR reconstitution in vitro. It is suggested that the function of Grx might be of significance for the insertion of FeS cluster in proteins containing disulfides.